The present invention relates to a method of manufacturing a plugged honeycomb structure having a substrate of a porous article and trapping layers which are provided on the surfaces of the substrate and trap particulate matter.
In gas discharged from internal combustion engines such as a diesel engine, various combustion apparatuses, and the like, a large amount of particulate matter (PM) mainly containing soot is contained. Since environmental pollution is caused when the PM is released in the air with no change, a filter apparatus for trapping PM is mounted in a flow passage of the exhaust gas.
As the core filter element of such a filter apparatus, for example, a honeycomb structure which is plugged (plugged honeycomb structure) is used. The plugged honeycomb structure has a plurality of cells separated by partition walls of a ceramic porous article having a large number of pores and functioning as gas passages (to obtain a honeycomb structure), and one side opening ends and the other side opening ends of adjacent cells are alternately plugged. When the plugged honeycomb structure is canned to allow exhaust gas to flow into the structure from one opening end of a cell, the exhaust gas passes through the partition walls and flows out from the other side opening ends of adjacent cells, and PM is trapped by the partition walls at that time to purify the exhaust gas.
However, in such a plugged honeycomb structure, since PM enters the inside of the porous article (substrate) to clog the pores, pressure loss is prone to increase rapidly. Therefore, in order to try to suppress pressure loss, for example, each of JP-A-10-249124 and JP-A-2006-685 discloses a plugged honeycomb structure having a new structure or a filter element as the use of the plugged honeycomb structure. As the common characteristics of these plugged honeycomb structures (filter elements), trapping layers for trapping PM are provided on a porous article (substrate) to try to inhibit the PM from entering the inside of the substrate by the trapping layers and inhibit pressure loss from rising. In addition, each of JP-A-10-249124 and JP-A-2006-685 discloses a method for manufacturing the new plugged honeycomb structure.
JP-A-10-249124 aims to manufacture a filter element of a ceramic material, where a filter layer is unitarily provided on one side face of a porous article. The filter element does not have a dense intermediate layer at the boundary of the porous article and the bonding portion of the filter layer and has low pressure loss with little clogging. In order to achieve the aim, in JP-A-10-249124, ceramic particles are conveyed by means of a gas current and allowed to adhere to one side surface of a porous article, and moisture (water or steam) is imparted to the adhering ceramic particles to allow the ceramic particles to be adsorbed to the one side face of the porous article and to allow the particles to mutually aggregate moderately and adhere in a secondary particle state, and thereby the ceramic particles are inhibited from entering the pores of the porous article and forming a dense intermediate layer. Specifically, the pore size of the filter layer is made smaller than the average pore size of the porous article, and the average particle diameter of the ceramic particles (forming the filter layer) is made to be ½ to ⅔ of the average pore size of the porous article. In addition, the air and the ceramic particles are sucked by a blower to form a filter layer, and the ceramic particles adhere to the porous article due to the differential pressure on the upstream side and the downstream side of the porous article.
JP-A-2006-685 aims to provide a plugged honeycomb structure excellent in the balance between the pressure loss and the trapping efficiency when the structure is used as a filter and a manufacturing method thereof. In order to achieve the aim, JP-A-2006-685 stipulates that, as an object (plugged honeycomb structure), it has ceramic particles adhering to the surfaces of the partition walls, that the average particle diameter of the ceramic particles is at least 1.5 times the average pore size of the partition walls and 200 μm or less, that ceramic particles are distributed on the surfaces of the partition walls in the range of ½ of the length from the central axis to the outer periphery, that the partition walls have a porosity of 50% to 80% and an average pore size of 15 μm to 40 μm, and the like. In addition, as a manufacturing method, it stipulates that the ceramic particles are sent into the cells from at least one end face by a fluid (gas), that ceramic particles generated upon machining the honeycomb structure are used, and the like.
However, in the manufacturing methods disclosed in JP-A-10-249124 and JP-A-2006-685, the input raw material (ceramic particles) does not form trapping layers efficiently to waste the raw material, and long time is spent for forming membranes of a desired amount. In addition, in order not to waste the raw material, the equipment and the step for reuse are necessary. Further, upon membrane formation, when the amount of ceramic particles contained in the gas for conveying the ceramic particles is large, ceramic particles are prone to aggregate, and the ceramic particles do not deposit uniformly to form membranes in a biased state.